If he says to his wife “Your bed is haraam for me”
This wording may be understand as referring to zihaar (a jaahili form of divorce), or it may be understood as referring to talaaq (divorce), or it may be understood as an oath.
That depends on the intention of the husband who spoke these words, because he knows what he intended when he said that.
If what he meant was that his wife was haraam for him, like his mother’s back, then this is zihaar, and it is not permissible for him to approach his wife until he has offered the expiation for zihaar, which Allaah has mentioned in the verse where He said (interpretation of the meaning):
“And those who make unlawful to them (their wives) by Zihaar and wish to free themselves from what they uttered, (the penalty) in that case is the freeing of a slave before they touch each other. That is an admonition to you (so that you may not repeat such an ill thing). And Allaah is All‑Aware of what you do.
4. And he who finds not (the money for freeing a slave) must fast two successive months before they both touch each other. And he who is unable to do so, should feed sixty Masaakeen (poor). That is in order that you may have perfect faith in Allaah and His Messenger. These are the limits set by Allaah. And for disbelievers, there is a painful torment”
If he intended by these words that divorce (talaaq) should take place, then it counts as a divorce, If it is the first or second talaaq then he may take his wife back so long as her ‘iddah has not yet ended. If it is a third talaaq then she is not permissible for him until she has been married to another husband.
If he intended to prevent himself from sleeping in her bed, and he did not intend zihaar or talaaq, then it is an oath. If he breaks it, he must offer kafaarat yameen (expiation for breaking an oath), which Allaah mentions in the verse where He says (interpretation of the meaning):
“Allaah will not punish you for what is unintentional in your oaths, but He will punish you for your deliberate oaths; for its expiation (a deliberate oath) feed ten Masaakeen (poor persons), on a scale of the average of that with which you feed your own families, or clothe them or manumit a slave. But whosoever cannot afford (that), then he should fast for three days. That is the expiation for the oaths when you have sworn. And protect your oaths (i.e. do not swear much). Thus Allaah makes clear to you His Ayaat (proofs, evidences, verses, lessons, signs, revelations, etc.) that you may be grateful”
See the answer to question no. 45676 for detail on the rulings on the expiation for breaking an oath.
And Allaah knows best.